Malignant transformation by Rous sarcoma virus is induced by a virus-coded protein which plays no role in virus reproduction. Immunoselection techniques have been used to identify virus-specific proteins in a variety of avian and mammalian cells transformed by Rous sarcoma virus, and the characterization of a protein possibly responsible for the induction and maintenance of malignancy is in progress. The function of the protein has been examined using a virus mutant inducing temperature-dependent transformation. Several biochemical changes occur in cells shifted from a nontransforming to a transformation-inducing temperature, and these changes occur subsequent to an alteration in cellular membranes which allows the increased influx of sodium ions. These studies suggest that the unequivocal identification of a virus-coded transforming protein and the resolution of its function are attainable.